SOURCE

    function FirstNotRepeatingChar(str) {
      if (!str) {
        return -1;
      }
      let countMap = {};
      const array = str.split('');
      const length = str.length;
      for (let i = 0; i < length; i++) {
        const current = array[i];
        let count = countMap[current];
        if (count) {
          countMap[current] = count + 1;
        } else {
          countMap[current] = 1;
        }
      }
      for (let i = 0; i < length; i++) {
        if (countMap[array[i]] === 1) {
          return i;
        }
      }
      return -1;
    }